The present invention can be applied to a broad range of electrical devices, although it is especially effective in light-receiving elements such as photodiodes and solar cells. The background of the invention is described below with reference to solar cells as a specific example of the prior art.
A conventional solar cell structure with a p-type base has a negative electrode that is typically on the front side or sun side of the cell and a positive electrode on the back side. Radiation of an appropriate wavelength falling on a p-n junction of a semiconductor substrate serves as a source of external energy to generate hole-electron pairs in the substrate. Because of the potential difference which exists at a p-n junction, holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metallized, i.e., provided with metal electrodes that are electrically conductive.
Typically pastes are screen printed onto the solar cell substrate and fired to form the electrodes. For example a single crystal or multi-crystalline p-type silicon substrate has an n-type diffusion layer adjacent to the light-receiving front side surface. An insulating anti-reflection coating may be formed on the n-type diffusion layer. As shown in FIG. 1, the semiconductor substrate 101 has electrodes 102 on the front side. These electrodes are typically mainly composed of silver. The back side has a compound electrode comprising electrodes 103 mainly composed of aluminum, as well as electrodes 104 mainly composed of silver or some other solderable material. Although the electrode 103 may contain components other than aluminum, it is aluminum-based and will be referred to herein as an aluminum electrode. When the pastes are fired aluminum diffuses into the silicon substrate 101 to form a back surface field (BSF) layer 105. The back surface field serves to improve the energy conversion efficiency of the solar cell. The aluminum electrode 103 is not solderable. The solderable Ag-based electrodes 104 are printed in the gaps between areas of the non-solderable aluminum back side electrode. The solderable Ag-based electrodes 104 are necessary to serve as tabbing bus bars to which tabbing ribbons can be soldered. The areas under the tabbing bus bars 104 have no back surface field (BSF), as shown in FIG. 1, which results in up to 0.2% cell efficiency loss compared to a solar cell with a full area of back surface field.
In one embodiment, the object of this invention is to provide a paste that can be used to form a solderable electrode on portions of an aluminum back side electrode of a solar cell. The aluminum electrode will cover the entire surface of the back side and thus provide a full BSF. This eliminates a solar cell efficiency loss due to interrupted BSF under the silver-containing electrodes typically used to create back side electrodes. The solderable electrodes formed from the paste of the invention will serve as tabbing bus bars.